1. Field of the Invention
This invention relates to a new process for the production of polyisocyanates containing isocyanurate groups based on mixtures of particular aliphatic-cycloaliphatic diisocyanates with 1,6-diisocyanatohexane, the polyisocyanates produced by this process and their use as an isocyanate component in polyisocyanate polyaddition products, preferably polyurethane lacquers.
2. Description of the Prior Art
(Cyclo)aliphatic polyisocyanates containing isocyanurate groups are technically of the greatest interest as lacquer polyisocyanates. Whereas low molecular weight aliphatic diisocyanates cannot be used as such in lacquer binders because of their toxicological properties due to their still relatively high volatility, isocyanato-isocyanurates show a number of advantages. They are oligomers with a low vapor pressure which are substantially monomer-free. They have an isocyanate functionality of .gtoreq.3, so that a high degree of cross-linking can be achieved. As aliphatic polyisocyanates they produce good light-fastness of the coatings obtained therefrom.
With respect to the production of polyisocyanates containing isocyanurate groups, the most varied processes have become known. These known processes generally differ in the selection of trimerization catalysts and also in the selection of the polyisocyanates to be used for the trimerization reaction (c.f. e.g. GB-PS No. 1,391,066, GB-PS No. 1,386,399, DE-OS No. 2,325,826, DE-OS No. 2,616,415, DE-OS No. 2,806,731, DE-OS No. 2,901,479, DE-OS No. 3,100,262, DE-OS No. 3,219,608, EP-PS No. 17,998, EP-OS No. 33,581, EP-OS No. 57,653, EP-OS No. 89,297, EP-PS No. 82,987 or EP-OS No. 100,129).
All of these processes suffer in principle from a substantial disadvantage. In order to obtain isocyanto-isocyanurates which are of low viscosity or have good solubility in lacquer solvents, the trimerization reaction must be terminated at a relatively low degree of trimerization (degree of trimerization=percentage of the isocyanate groups present in the starting isocyanates which are trimerized) and, further, in order to obtain a trimerization product with a low monomer content, a costly thin layer distillation must be carried out. Otherwise, in addition to the desired low viscosity monomeric isocyanurates with good solubility, relatively large quantities of oligoisocyanurates are formed (polyisocyanates which have more than one isocyanurate ring per molecule). This is due to the fact that the isocyanate groups of the starting diisocyanates generally have similar or only slightly different reactivities, such that both isocyanate groups of the starting diisocyanate are trimerized.
According to German Patent Application No. P 35 07 719.0 lacquer polyisocyanates containing isocyanurate groups based on aliphatic-cycloaliphatic diisocyanates may be produced without premature termination of the trimerization reaction due to the extremely different reactivities of the isocyanate groups of these special diisocyanates. The isocyanato-isocyanurates which can be obtained according to this application substantially contain the tris-isocyanato-isocyanurates corresponding to the diisocyanates used and produce valuable binder components for two-component polyurethane lacquers.
It has now been surprisingly found that a substantially analogous trimerization of mixtures of these special aliphatic-cycloaliphatic diisocyanates with 1,6-diisocyanatohexane results in isocyanato-isocyanurates which are more suitable as a binder component in two-component polyurethane lacquers than the products of the prior application. In particular the lacquer films produced with the products according to the invention have a higher elasticity than analogous lacquers based on the isocyanato-isocyanurates according to the prior application.